Fourth Power just closed a $20 million Series A Plus to build what could be the energy industry's next breakthrough - thermal batteries that store electricity in molten tin at 2,400°C. The Cambridge startup claims its sci-fi approach will deliver grid-scale storage at $25 per kilowatt-hour, potentially one-tenth the cost of lithium-ion while lasting twice as long.
Fourth Power just turned energy storage into something out of a James Bond film. The Cambridge startup closed a $20 million Series A Plus round to build thermal batteries that store electricity using molten tin heated to a scorching 2,400°C - hot enough to melt copper twice over. Munich Re Ventures led the round, with Breakthrough Energy Ventures and DCVC participating, bringing the company's total funding to $39 million since its previous $19 million Series A in 2023.
The timing couldn't be better. Grid operators are scrambling for long-duration storage solutions as solar and wind power floods the system during peak hours but disappears when the sun sets. Current lithium-ion batteries typically discharge for 4 hours max - not enough to bridge the gap between afternoon solar peaks and evening demand spikes.
Here's where Fourth Power's approach gets interesting. Instead of cramming more lithium into boxes, they're essentially building thermal vaults. Electricity from the grid heats carbon blocks inside argon-filled chambers. When power's needed, the system pumps molten tin through graphite pipes - the only material that can handle those extreme temperatures without breaking the bank. Special thermophotovoltaic cells then convert the infrared radiation back into electricity.
"Our projections are for the first-of-a-kind — the first ones to market — that they'll be cost competitive," CEO and co-founder Arvin Ganesan told TechCrunch exclusively. "Our comps for these markets are very high."
Those "comps" include peaking natural gas plants, which utilities fire up when demand spikes and renewable sources fall short. Fourth Power's batteries can discharge for 8+ hours daily - twice as long as most grid-scale lithium systems. The insulation, made from petroleum coke (basically oil refining waste), keeps the system stable while losing just 1% of stored energy per day.
The real game-changer is the projected cost: $25 per kilowatt-hour at scale. That's roughly one-tenth the cost of lithium-ion batteries and could make renewable energy viable for 24/7 baseload power. "There's not a lot of moving pieces. There's not a lot of players in the supply chain, which means that you can get to cost targets in a fairly straightforward way," Ganesan explained.
